The present invention relates to a picture signal quantizing circuit for quantizing with respect to multiple levels an analog image or picture signal indicative of an image or picture read from a document by causing an analog-to-digital (AD) converter to convert the picture signal into a digital picture signal having a plurality of bits.
Generally, a picture signal quantizing circuit of the type described comprises a parallel comparison type AD converter which is made up of a reference voltage divider consisting of a series connection of 2.sup.n ladder resistors, a comparator circuit consisting of (2.sup.n -1) high speed comparators adapted to compare outputs from the junctions between the ladder resistors and an analog picture signal, and an encoder for preparing a digital signal as an n-bit binary code in response to outputs of the comparators. The analog picture signal is converted into the n-bit digital picture signal within the range of an upper limit reference voltage applied to one end of the series connection and a lower limit reference voltage applied to the other end.
Difficulty has been experienced in such a prior art quantizing circuit in dither processing the multi-level quantized picture signal into two levels for reproduction of halftone. Should the difference between the highest and lowest reference voltages be reduced to narrow the AD conversion range for an analog picture signal and, thereby, accentuate the contrast, the range available for the reproduction of halftone would also be reduced resulting in poor reproducibility of photographs and other halftone images. Conversely, should the AD conversion range be enlarged to attain better reproducibility of halftone, the quantization would become unfeasible for picture signals representative of characters and the like.